Glutathione is a ubiquitous tripeptide found in high concentration in normal tissue and is frequently elevated in tumors. Glutathione and its oxidized disulfide form the primary reduction/oxidation (redox) buffer in the cell. The cellular redox balance controls gene expression, differentiation, proliferation and apoptosis. In normal tissue, glutathione protects the cell from environmental stress and the cancer cell has adapted this defense mechanism to shield tumor cells from the effects of therapy. In cell culture, our laboratory has shown that glutathione metabolism is consistently increased in drug-resistant cancers. These findings are supported by a number of clinical studies relating glutathione metabolism to patient survival. However the clinical studies rely upon invasive repeated sampling of the tumor tissue since methods to monitor glutathione metabolism in intact rumors are lacking. Over the past two years, as part of an NTH-funded pilot project, we succeeded in developing a magnetic resonance method combined with stable isotope infusion to monitor glutathione metabolism in vivo. This proposal builds upon the feasibility established in the pilot project and plans to Use these methods to monitor glutathione metabolism non-invasively in R3230 mammary, 9L glioma and FSA fibrosarcoma tumors and drug-resistant variants in rats. Glutathione metabolism will be assessed in relation to tumor growth rate, vascularization and expression of key enzymes involved in redox metabolism. Since a drop in intracellular glutathione levels is one of the earliest events in apoptosis, therapy-induced changes in glutathione metabolism detected by magnetic resonance will be an early indicator of tumor response. Since altered redox balance appears to be a universal characteristic of proliferating cells in general and cancer in particular, methods to non-invasively monitor redox metabolism offers new insight into a process critical to planning and evaluating cancer therapy.